...wondered if there was anyway to deselect a thread, so that it no longer appears when you use the "view your posts" feature. After all, when you've been a member for awhile, the list gets somewhat long, and it would be nice to be able to kill old inactive threads from the list.

There's no way to do that. You can however select threads you like, add them to your subscriptions (go to your Control Panel and choose Manage Subscriptions to see which threads you have). and control which ones you see regularly that way. The View Your posts lists all the threads you've posted to at some point.